The invention relates generally to electrodes for electric plasma discharge devices.
Low-pressure metal halide electric discharge plasmas have the potential to replace the mercury electric discharge plasma in conventional fluorescent lamps. However, many conventionally used electron emission materials, such as barium oxide, are not chemically stable in the presence of a metal halide plasma. Although the applicants do not wish to be bound by any theory, it is believed, for example, that a barium oxide (BaO) electron emissive material may react with a metal halide (MeX, wherein Me is the metal and X is the halogen) vapor, such as indium iodide vapor present in a discharge medium, leading to the formation of barium halide (BaX) vapor and a condensed metal oxide (MeO). Other conventionally used electron emission materials, such as calcium oxide and strontium oxide, may be less reactive with metal halide vapors. However, most electron emissive materials are expected to react with the metal halide vapor to some degree.
Even in conventional mercury-based fluorescent lamps, reactions which occur between the electrode material and the discharge material (mercury) are disadvantageous. In particular, mercury can react or amalgamate with electron emissive materials such as barium oxide, or with reaction products of the emissive materials. It is believed that electrode material deposits are formed on the inner wall of the lamp, as the lamp ages, and the mercury in the discharge amalgamates with the electrode material that has deposited on the wall. After this reaction or amalgamation, the mercury is more strongly bound and cannot evaporate as easily from the wall during normal operation, and hence is effectively removed from participation in the light-generation mechanism of the lamp. Undesirably, additional mercury must be placed into the lamp during its manufacture, to compensate for the mercury that is effectively lost to reaction or amalgamation, and ensure that the lamp meets its rated operational life. The reaction and amalgamation of mercury can be managed through the use of shields, which can provide both a physical as well as a chemical barrier to the loss of mercury, but the addition of shields also adds undesirably to the cost and complexity of the lamp.
Metal electrodes, such as tungsten electrodes, without electron emissive material coatings, are known in the art for high-pressure high-intensity-arc-discharge (HID) lamps. Some non-thermionic metal electrodes are also known in the art for low-pressure discharge plasmas, but only when the electrodes are relatively cold, below their thermionic electron emission temperature (for example, less than 1500 degree K). In the case of non-thermionic metal electrodes, the electrons are emitted from the electrode by “secondary electron emission” (in response to an incident high-energy ion, where typically the ion energy is 100-150 electron volts), or photoelectron emission (in response to a photon of sufficiently high energy). Such “cold cathodes” are used in neon signs and in “cold cathode fluorescent lamps” for display backlights, but because of the high cathode-fall voltage, the lamp discharge voltage is typically very high (>1 kV) to achieve good device efficiency. For general lighting, hot-cathode fluorescent lamps are commonly used instead of cold-cathode lamps, because of their higher efficiency and lower operating voltage.
Therefore there is a need for an electrode design which addresses one or more of the foregoing problems with electrodes used in low-pressure plasma discharge devices.